Initial import

1 files changed, 225 insertions, 0 deletions

diff --git a/arch/x86/include/asm/rwsem.h b/arch/x86/include/asm/rwsem.h
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index 000000000..cad82c9c2
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+++ b/arch/x86/include/asm/rwsem.h
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+/* rwsem.h: R/W semaphores implemented using XADD/CMPXCHG for i486+
+ *
+ * Written by David Howells (dhowells@redhat.com).
+ *
+ * Derived from asm-x86/semaphore.h
+ *
+ *
+ * The MSW of the count is the negated number of active writers and waiting
+ * lockers, and the LSW is the total number of active locks
+ *
+ * The lock count is initialized to 0 (no active and no waiting lockers).
+ *
+ * When a writer subtracts WRITE_BIAS, it'll get 0xffff0001 for the case of an
+ * uncontended lock. This can be determined because XADD returns the old value.
+ * Readers increment by 1 and see a positive value when uncontended, negative
+ * if there are writers (and maybe) readers waiting (in which case it goes to
+ * sleep).
+ *
+ * The value of WAITING_BIAS supports up to 32766 waiting processes. This can
+ * be extended to 65534 by manually checking the whole MSW rather than relying
+ * on the S flag.
+ *
+ * The value of ACTIVE_BIAS supports up to 65535 active processes.
+ *
+ * This should be totally fair - if anything is waiting, a process that wants a
+ * lock will go to the back of the queue. When the currently active lock is
+ * released, if there's a writer at the front of the queue, then that and only
+ * that will be woken up; if there's a bunch of consequtive readers at the
+ * front, then they'll all be woken up, but no other readers will be.
+ */
+
+#ifndef _ASM_X86_RWSEM_H
+#define _ASM_X86_RWSEM_H
+
+#ifndef _LINUX_RWSEM_H
+#error "please don't include asm/rwsem.h directly, use linux/rwsem.h instead"
+#endif
+
+#ifdef __KERNEL__
+#include <asm/asm.h>
+
+/*
+ * The bias values and the counter type limits the number of
+ * potential readers/writers to 32767 for 32 bits and 2147483647
+ * for 64 bits.
+ */
+
+#ifdef CONFIG_X86_64
+# define RWSEM_ACTIVE_MASK		0xffffffffL
+#else
+# define RWSEM_ACTIVE_MASK		0x0000ffffL
+#endif
+
+#define RWSEM_UNLOCKED_VALUE		0x00000000L
+#define RWSEM_ACTIVE_BIAS		0x00000001L
+#define RWSEM_WAITING_BIAS		(-RWSEM_ACTIVE_MASK-1)
+#define RWSEM_ACTIVE_READ_BIAS		RWSEM_ACTIVE_BIAS
+#define RWSEM_ACTIVE_WRITE_BIAS		(RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
+
+/*
+ * lock for reading
+ */
+static inline void __down_read(struct rw_semaphore *sem)
+{
+	asm volatile("# beginning down_read\n\t"
+		     LOCK_PREFIX _ASM_INC "(%1)\n\t"
+		     /* adds 0x00000001 */
+		     "  jns        1f\n"
+		     "  call call_rwsem_down_read_failed\n"
+		     "1:\n\t"
+		     "# ending down_read\n\t"
+		     : "+m" (sem->count)
+		     : "a" (sem)
+		     : "memory", "cc");
+}
+
+/*
+ * trylock for reading -- returns 1 if successful, 0 if contention
+ */
+static inline int __down_read_trylock(struct rw_semaphore *sem)
+{
+	long result, tmp;
+	asm volatile("# beginning __down_read_trylock\n\t"
+		     "  mov          %0,%1\n\t"
+		     "1:\n\t"
+		     "  mov          %1,%2\n\t"
+		     "  add          %3,%2\n\t"
+		     "  jle	     2f\n\t"
+		     LOCK_PREFIX "  cmpxchg  %2,%0\n\t"
+		     "  jnz	     1b\n\t"
+		     "2:\n\t"
+		     "# ending __down_read_trylock\n\t"
+		     : "+m" (sem->count), "=&a" (result), "=&r" (tmp)
+		     : "i" (RWSEM_ACTIVE_READ_BIAS)
+		     : "memory", "cc");
+	return result >= 0 ? 1 : 0;
+}
+
+/*
+ * lock for writing
+ */
+static inline void __down_write_nested(struct rw_semaphore *sem, int subclass)
+{
+	long tmp;
+	asm volatile("# beginning down_write\n\t"
+		     LOCK_PREFIX "  xadd      %1,(%2)\n\t"
+		     /* adds 0xffff0001, returns the old value */
+		     "  test " __ASM_SEL(%w1,%k1) "," __ASM_SEL(%w1,%k1) "\n\t"
+		     /* was the active mask 0 before? */
+		     "  jz        1f\n"
+		     "  call call_rwsem_down_write_failed\n"
+		     "1:\n"
+		     "# ending down_write"
+		     : "+m" (sem->count), "=d" (tmp)
+		     : "a" (sem), "1" (RWSEM_ACTIVE_WRITE_BIAS)
+		     : "memory", "cc");
+}
+
+static inline void __down_write(struct rw_semaphore *sem)
+{
+	__down_write_nested(sem, 0);
+}
+
+/*
+ * trylock for writing -- returns 1 if successful, 0 if contention
+ */
+static inline int __down_write_trylock(struct rw_semaphore *sem)
+{
+	long result, tmp;
+	asm volatile("# beginning __down_write_trylock\n\t"
+		     "  mov          %0,%1\n\t"
+		     "1:\n\t"
+		     "  test " __ASM_SEL(%w1,%k1) "," __ASM_SEL(%w1,%k1) "\n\t"
+		     /* was the active mask 0 before? */
+		     "  jnz          2f\n\t"
+		     "  mov          %1,%2\n\t"
+		     "  add          %3,%2\n\t"
+		     LOCK_PREFIX "  cmpxchg  %2,%0\n\t"
+		     "  jnz	     1b\n\t"
+		     "2:\n\t"
+		     "  sete         %b1\n\t"
+		     "  movzbl       %b1, %k1\n\t"
+		     "# ending __down_write_trylock\n\t"
+		     : "+m" (sem->count), "=&a" (result), "=&r" (tmp)
+		     : "er" (RWSEM_ACTIVE_WRITE_BIAS)
+		     : "memory", "cc");
+	return result;
+}
+
+/*
+ * unlock after reading
+ */
+static inline void __up_read(struct rw_semaphore *sem)
+{
+	long tmp;
+	asm volatile("# beginning __up_read\n\t"
+		     LOCK_PREFIX "  xadd      %1,(%2)\n\t"
+		     /* subtracts 1, returns the old value */
+		     "  jns        1f\n\t"
+		     "  call call_rwsem_wake\n" /* expects old value in %edx */
+		     "1:\n"
+		     "# ending __up_read\n"
+		     : "+m" (sem->count), "=d" (tmp)
+		     : "a" (sem), "1" (-RWSEM_ACTIVE_READ_BIAS)
+		     : "memory", "cc");
+}
+
+/*
+ * unlock after writing
+ */
+static inline void __up_write(struct rw_semaphore *sem)
+{
+	long tmp;
+	asm volatile("# beginning __up_write\n\t"
+		     LOCK_PREFIX "  xadd      %1,(%2)\n\t"
+		     /* subtracts 0xffff0001, returns the old value */
+		     "  jns        1f\n\t"
+		     "  call call_rwsem_wake\n" /* expects old value in %edx */
+		     "1:\n\t"
+		     "# ending __up_write\n"
+		     : "+m" (sem->count), "=d" (tmp)
+		     : "a" (sem), "1" (-RWSEM_ACTIVE_WRITE_BIAS)
+		     : "memory", "cc");
+}
+
+/*
+ * downgrade write lock to read lock
+ */
+static inline void __downgrade_write(struct rw_semaphore *sem)
+{
+	asm volatile("# beginning __downgrade_write\n\t"
+		     LOCK_PREFIX _ASM_ADD "%2,(%1)\n\t"
+		     /*
+		      * transitions 0xZZZZ0001 -> 0xYYYY0001 (i386)
+		      *     0xZZZZZZZZ00000001 -> 0xYYYYYYYY00000001 (x86_64)
+		      */
+		     "  jns       1f\n\t"
+		     "  call call_rwsem_downgrade_wake\n"
+		     "1:\n\t"
+		     "# ending __downgrade_write\n"
+		     : "+m" (sem->count)
+		     : "a" (sem), "er" (-RWSEM_WAITING_BIAS)
+		     : "memory", "cc");
+}
+
+/*
+ * implement atomic add functionality
+ */
+static inline void rwsem_atomic_add(long delta, struct rw_semaphore *sem)
+{
+	asm volatile(LOCK_PREFIX _ASM_ADD "%1,%0"
+		     : "+m" (sem->count)
+		     : "er" (delta));
+}
+
+/*
+ * implement exchange and add functionality
+ */
+static inline long rwsem_atomic_update(long delta, struct rw_semaphore *sem)
+{
+	return delta + xadd(&sem->count, delta);
+}
+
+#endif /* __KERNEL__ */
+#endif /* _ASM_X86_RWSEM_H */